This invention relates to formulations for removing mercury and other pollutants from fluid streams. Specifically, it provides formulations for mercury removal from gaseous streams known as combustion or flue gases of fossil and derived fuels. The flue gases of fossil and derived fuels contain mercury and other pollutants in small quantities because such fuels contain mercury and other pollutants in them. Sometimes, the process of combustion itself produces some pollutants such as carbon monoxide, oxides of nitrogen, dioxin, etc. In addition, some process streams may also contain mercury, other pollutants and substances that may require removal for one reason or the other.
The invention disclosed herein can be used to remove mercury and other pollutants from streams containing them as long as the temperature of the stream is above 200° F. The present invention is most suitable for the removal of mercury and other pollutants from combustion gases in boilers equipped with particulate control devices. The combustion of fossil and derived fuels, particularly solid fuels, and more particularly in the USA, is carried out in such boilers. For the present invention, the fossil fuels are defined as fuels such as coal, lignite, peat and fuel oil and the derived fuels are defined as municipal, industrial and pharmaceutical wastes and refuge.
Environmental considerations require that emissions of hazardous pollutants such as mercury and dioxin be contained. Most US coals and municipal refuge contain mercury in them which is released in the form of elemental and oxidized mercury with the combustion gases. Dioxin is generally produced during the combustion process from the precursor components present in the fossil or derived fuels. Unless the mercury and the precursors leading to the formation of dioxin are removed from the fuel prior to its combustion, they become a component of the gases produced by the combustion of the fuel and become extremely difficult to remove effectively and economically by known processes.
Coal (a term utilized in here to describe solid fuels such as bituminous and sub-bitumnous coals, anthracite, lignite and peat) is one of the most important fuels for producing power. It is burned in boilers all over the world to produce steam and electrical power. Power plants in the USA is estimated to burn more than 900 million tons a year of coal.
Coals contain many impurities including ash, sulfur, mercury, arsenic, selenium, berillium, boron, etc. When coal is burned in a furnace it is converted to carbon dioxide and water producing heat. The impurity such as ash remains behind as a residue while the majority of other impurities such as sulfur, mercury, arsenic, etc. leave with the combustion gases, also known as flue gases.
Depending upon the firing practices utilized, the ash is removed as bottom ash or as a combination of bottom and fly ash. The fly ash is that portion of the ash that becomes entrained in the combustion gases and moves around with them into the various parts of the boiler or combustion systems. Since the ash is entrained with the combustion gases, it is removed from the combustion gases before the gases are discharged into the atmosphere through chimneys or stacks. The separation of the entrained or the fly ash from the combustion gases is accomplished by utilizing particulate control devices such as cyclones, electrostatic precipitators, bag houses or their combinations.
Emissions of mercury from power plants, though minuscule in mass compared to ash and oxides of sulfur and nitrogen commonly referred to SOx and NOx, are targeted for control due to its tendency to bioaccumulate, and its potency as a neurotoxin.
The mercury is emitted from the stacks with the combustion gases in the form of elemental and oxidized mercury. The ratio between the elemental and the oxidized forms depends upon the type of the coal being burned and the equipment it is burned in. The ratio of the oxidized to the elemental (un-oxidized) forms of mercury when burning bituminous or eastern coals is higher than when burning sub-bituminous or western coals. The higher ratio when burning bituminous coals is believed due to the presence of a higher level of chlorides in the bituminous than sub-bituminous coals.
Many novel and unique methods are currently being evaluated to control the emission of mercury from the stack gases. Most of the processes require injection of a mercury specific sorbent(s) into the combustion gas stream. The sorbent is injected prior to the particulate control device(s) so that the sorbent containing the adsorbed mercury is removed by the particulate control device(s) together with the fly ash.
Among the sorbents tried have been powdered activated carbon, various chars, clays, zeolites, different types of fly ash, fly ash enriched with unburned carbon, etc. However, in the USA, as it is practiced commercially in the countries of Europe and Asia, powdered activated carbon is one of the most effective sorbents for mercury removal. The powder activated carbon is blown in by compressed air into the combustion gases upstream of the particulate control device where the gaseous temperature ranges between 250 and 800° F., depending on the particulate control device type. In the case of cold side electrostatic precipitators and bag houses the temperatures range is between 250 and 400° F. The hotside electrostatic precipitators operate around 800° F. The powder carbon works best when the gas temperatures are low and for that reason even in the coldside applications, sometimes, the flue gases are cooled by injecting a fine spray of water.
Sometimes, the carbon is specially modified by adding sulfur, iodine, chloride, etc. to make it more suitable to remove the mercuric form of mercury. Such specificity is introduced in the carbon either during its manufacturing or as a separate step after the carbon has been manufactured.
The ineffectiveness of carbon at high temperatures requiring gas cooling and introduction of specificity to the carbon adds to the overall cost of mercury removal. In addition, the costs of commercial carbons is a major factor in keeping the mercury removal costs unacceptable.
Unlike the aforementioned methods of mercury control, use of the compositions of the present invention provides an effective, effecient, and low cost means for controlling mercury and other pollutant emissions with very little limitations. The sorbents are generated in very fine, expected to be finer than pulverized, powdered form of activated carbon. The sorbents with or without the specificity producing components as described herein, are produced in-situ and are thus freshly produced, expected to be much more effective than powdered carbon which has been exposed to air and moisture during its storage and transportation to the site where the controls are needed. Moreover, use of these invented compositions fills an important need by reducing these emissions simultaneously from flue and process gases produced from the combustion of fossil or derived fuels. Because of these desirable characteristics, the present invention constitutes a significant advancement over prior control techniques.